In a typical air-conditioning apparatuses, there are cases in which stagnation (hereinafter also referred to as “accumulation”) of a refrigerant occurs in a compressor while the apparatus is stopped.
The stagnant refrigerant in the compressor dissolves in lubricant in the compressor. This reduces the concentration of the lubricant, and thus reduces the viscosity of the lubricant.
If the compressor is started under such a condition, the lubricant having low viscosity is supplied to the rotating shaft and the compression unit of the compressor. This may result in burnout of sliding portions and the like in the compressor due to insufficient lubrication.
Furthermore, the stagnant refrigerant in the compressor raises the liquid level in the compressor. This increases the starting load of a motor for driving the compressor. The increased starting load may be identified as an overcurrent at the start-up of the air-conditioning apparatus. Thus, the air-conditioning apparatus may fail to start.
In order to solve these problems, a measure has been taken to prevent accumulation of a refrigerant in the compressor by heating the compressor while the compressor is stopped.
One method of heating the compressor is to energize an electric heater wound around the compressor. Another method is to apply a high-frequency, low-voltage current to a coil of the motor in the compressor. With this method, without rotating the motor, the compressor is heated with Joule heat generated in the coil.
However, since the compressor is heated in order to prevent stagnation of a refrigerant in the compressor while the compressor is stopped, power is consumed even while the air-conditioning apparatus is stopped.
As a countermeasure against this problem, there has been proposed a technique that “detects an outside air temperature, changes the time length or the voltage of energization from an inverter device to a motor coil in accordance with the outside air temperature, and controls the temperature of the compressor to be substantially constant regardless of changes in the outside air temperature” (see Patent Literature 1, for example.)
There has been also proposed a device that “includes saturation temperature calculating means that calculates a saturation temperature of a refrigerant in a compressor on the basis of a pressure detected by pressure detection means; and control means that compares the calculated saturation temperature with a detection temperature detected by temperature detection means, determines a state in which the refrigerant is easily condensed, and controls the heater so as to heat the compressor in the case where the compressor is stopped and the refrigerant in the compressor is in the state in which the refrigerant is easily condensed” (see Patent Literature 2, for example).